1. Field of the Invention
This invention relates to the reduction of pollutants in exhaust gases from internal combustion engines and more particularly to an improved system for metering reducing agents into exhaust gases for reducing pollutants.
2. Description of the Prior Art
An established method for reducing pollutants, particularly for reducing nitrogen oxides in the exhaust gas of internal combustion engines operated with lean mixtures, is reducing the nitrogen oxides by means of suitable chemical reducing agents. It is a feature common to the known methods that fluid (that is, liquid or gaseous), pollutant-reducing media are injected into an exhaust system of an internal combustion engine and there react with the pollutants in the exhaust gases and convert them chemically. A metering system for metering such fluid, pollutant-reducing media is therefore proposed below that can be employed for reducing pollutants in the exhaust gases of motor vehicles.
In what is known as the ammonia-SCR method, especially in the utility vehicle field, ammonia is needed as a reducing agent in the exhaust gas upstream of a corresponding reduction catalytic converter. In current methods, by blowing an aqueous urea-air aerosol into the engine exhaust gas by means of compressed air, the actual reducing agent, ammonia, is released by thermolysis and ensuing catalyzed hydrolysis.
For possible use in passenger cars, however, because of its high system complexity (costs, installation space, compressed air supply), this method with blown-in compressed air cannot be considered very appropriate for the market.
A further method for avoiding the dependency on compressed air provides for injecting a urea solution (AdBlue) into the engine exhaust gases. For that purpose, pumps and self-preparing valves are for instance used for feeding pressure to the AdBlue.
In this respect, valves are often used of the kind known from gasoline injection, which have various disadvantages. In these valves, the necessity of cooling the (usually electrical) valves that are mounted on the hot exhaust system, presents difficulties. Moreover, the winter functioning of such systems must be considered problematic. At temperatures below −11° C., the conventional urea-water solution freezes and expands. Hence to attain winter functioning of such valves and other system components affected (pumps, pressure regulation, and so forth), considerable engineering effort that accordingly increases the system complexity must be tolerated (for instance, ice-pressure-proof construction, reaspirating pumps, and so forth).
In German Patent DE 196 46 643 C1, a system for nitrogen oxide reducing agent injection into an exhaust gas flow is described. The reducing agent is injected controllably into the exhaust gas flow in the form of fine streams through a plurality of fine nozzle openings, by means of local, clocked overpressure generation. As the nozzles, piezoelectricaily controlled nozzles are used, similar to the nozzles in piezoelectric ink jet print heads or nozzles with clocked heating resistor elements, similar to known bubble jet ink jet print heads. The assembly described in DE 196 46 643 C1, however, is complicated in terms of equipment, vulnerable to malfunction especially with regard to the aggressive fluids involved, and expensive.